W2k3 randomly dropping clients?

Problem:
Users report they will randomly lose connection to the programs they are working on that are hosted on a our server. The two programs they are having problems with are Caselle as well as QuickBooks Pro 07.

Things I have tried:
Update server NIC drivers
Checked for problems with recent updates
performed updates for all software

Note: Honestly I have no idea how to even go about this problem. Nothing looks weird in the event log. The server had been running for 10days before it randomly decided to act up. Any help?